Analysis of Dependence of Breakdown Voltage on Gate-Drain Distance in AlGaN/GaN HEMTs With High-k Passivation Layer

A 2-D analysis of OFF-state breakdown characteristics of AlGaN/GaN HEMTs with a high- ${k}$ passivation layer is performed as a function of gate-to-drain distance ${L} _{\text {GD}}$ . The relative permittivity of the passivation layer $\varepsilon _{\text {r}}$ is changed from 1 to 60, and ${L} _{\text {GD}}$ is changed from 1.5 to $10~\mu \text{m}$ . It is shown that, in all cases with different ${L} _{\text {GD}}$ , the breakdown voltage ${V} _{\text {br}}$ increases as $\varepsilon _{\text {r}}$ increases. When a deep-acceptor density in an Fe-doped buffer layer ${N} _{\text {DA}}$ is $10^{{17}}$ cm $^{-{3}}$ and the gate length is $0.3~\mu \text{m}$ , ${V} _{\text {br}}$ is determined by buffer leakage current at $\varepsilon _{\text {r}} \ge30$ before impact ionization dominates. Hence, ${V} _{\text {br}}$ is similar at ${L} _{\text {GD}} =3$ – $10~\mu \text{m}$ , and the increase rate in ${V} _{\text {br}}$ from ${L} _{\text {GD}} = 1.5\,\,\mu \text{m}$ is about 50% even at $\varepsilon _{\text {r}} =60$ . However, when ${N} _{\text {DA}}$ is $2\times 10^{{17}}$ cm $^{-{3}}$ , ${V} _{\text {br}}$ is determined by impact ionization of carriers even at $\varepsilon _{\text {r}} \ge30$ because the buffer leakage current is reduced. ${V} _{\text {br}}$ becomes about 500, 930, 1360, and 1650 V for ${L} _{\text {GD}} =1.5$ , 3, 5, and $7~\mu $ m, respectively, at $\varepsilon _{\text {r}} =60$ . These voltages correspond to gate-to-drain average electric fields of about 3.3, 3.1, 2.7, and 2.3 MV/cm, respectively. Particularly, for short ${L} _{\text {GD}}$ , the electric field profiles between the gate and the drain are rather uniform. However, in the case of ${L} _{\text {GD}} = 10\,\,\mu \text{m}$ , ${V} _{\text {br}}$ is about the same as that (1650 V) of ${L} _{\text {GD}} = 7\,\,\mu \text{m}$ , suggesting that the electric field at the drain edge of the gate becomes a critical value before the high electric field region extends to the drain enough. This may be a limitation to increase ${V} _{\text {br}}$ by using a high- ${k}$ passivation layer in this case. However, it can be said that, to improve ${V} _{\text {br}}$ further at long ${L} _{\text {GD}}$ , such as $10~\mu \text{m}$ , the combination of field plate or using a higher $\varepsilon _{\text {r}}$ material may be effective because both of them decrease the electric field at the drain edge of the gate.